A.B. Martins-Bach

Metabolic profile of dystrophic mdx mouse muscles analyzed with in vitro magnetic resonance spectroscopy (MRS)

Duchenne muscular dystrophy (DMD) is a recessive X-linked form of muscular dystrophy characterized by progressive and irreversible degeneration of the muscles. The mdx mouse is the classical animal model for DMD, showing similar molecular and protein defects. The mdx mouse, however, does not show significant muscle weakness, and the diaphragm muscle is significantly more degenerated than skeletal muscles. In this work, (1)H magnetic resonance spectroscopy (MRS) was used to study the metabolic profile of quadriceps and diaphragm muscles from mdx and control mice. Using principal components analysis (PCA), the animals were separated into groups according to age and lineages. The classification was compared to histopathological analysis. Among the 24 metabolites identified from the nuclear MR spectra, only 19 were used by the PCA program for classification purposes. These can be important key biomarkers associated with the progression of degeneration in mdx muscles and with natural aging in control mice. Glutamate, glutamine, succinate, isoleucine, acetate, alanine and glycerol were increased in mdx samples as compared to control mice, in contrast to carnosine, taurine, glycine, methionine and creatine that were decreased. These results suggest that MRS associated with pattern recognition analysis can be a reliable tool to assess the degree of pathological and metabolic alterations in the dystrophic tissue, thereby affording the possibility of evaluation of beneficial effects of putative therapies.

Structural and Functional Alterations of Skeletal Muscle Microvasculature in Dystrophin-Deficient mdx Mice.

Duchenne muscular dystrophy (DMD) is a progressive neuromuscular disease, caused by an absence of dystrophin, inevitably leading to death. Although muscle lesions are well characterized, blood vessel alterations that may have a major impact on muscle regeneration remain poorly understood. Our aim was to elucidate alterations of the vascular network organization, taking advantage of Flk1(GFP/+) crossed with mdx mice (model for human DMD where all blood vessels express green fluorescent protein) and functional repercussions using in vivo nuclear magnetic resonance, combining arterial spin-labeling imaging of perfusion, and (31)P-spectroscopy of phosphocreatine kinetics. For the first time, our study focused on old (12-month-old) mdx mice, displaying marked chronic muscle lesions, similar to the lesions observed in human DMD, in comparison to young-adult (3-month-old) mdx mice displaying only mild muscle lesions with no fibrosis. By using an original approach combining a specific animal model, state-of-the-art histology/morphometry techniques, and functional nuclear magnetic resonance, we demonstrated that the microvascular system is almost normal in young-adult in contrast to old mdx mice, displaying marked microvessel alterations, and the functional repercussions on muscle perfusion and bioenergetics after a hypoxic stress vary depending on stage of pathology. This original approach clarifies disease evolution and paves the way for setting up new diagnostic markers or therapeutic strategies.

Dmdmdx/Largemyd: a new mouse model of neuromuscular diseases useful for studying physiopathological mechanisms and testing therapies

SUMMARY Although muscular dystrophies are among the most common human genetic disorders, there are few treatment options available. Animal models have become increasingly important for testing new therapies prior to entering human clinical trials. The Dmdmdx mouse is the most widely used animal model for Duchenne muscular dystrophy (DMD), presenting the same molecular and protein defect as seen in humans with the disease. However, this mouse is not useful for clinical trials because of its very mild phenotype. The mouse model for congenital myodystrophy type 1D, Largemyd, harbors a mutation in the glycosyltransferase Large gene and displays a severe phenotype. To help elucidate the role of the proteins dystrophin and LARGE in the organization of the dystrophin-glycoprotein complex in muscle sarcolemma, we generated double-mutant mice for the dystrophin and LARGE proteins. The new Dmdmdx/Largemyd mouse model is viable and shows a severe phenotype that is associated with the lack of dystrophin in muscle. We tested the usefulness of our new mouse model for cell therapy by systemically injecting them with normal murine mesenchymal adipose stem cells (mASCs). We verified that the mASCs were hosted in the dystrophic muscle. The new mouse model has proven to be very useful for the study of several other therapies, because injected cells can be screened both through DNA and protein analysis. Study of its substantial muscle weakness will also be very informative in the evaluation of functional benefits of these therapies.

Quantitative T2 Combined with Texture Analysis of Nuclear Magnetic Resonance Images Identify Different Degrees of Muscle Involvement in Three Mouse Models of Muscle Dystrophy: mdx, Largemyd and mdx/Largemyd

Quantitative nuclear magnetic resonance imaging (MRI) has been considered a promising non-invasive tool for monitoring therapeutic essays in small size mouse models of muscular dystrophies. Here, we combined MRI (anatomical images and transverse relaxation time constant—T2—measurements) to texture analyses in the study of four mouse strains covering a wide range of dystrophic phenotypes. Two still unexplored mouse models of muscular dystrophies were analyzed: The severely affected Largemyd mouse and the recently generated and worst double mutant mdx/Largemyd mouse, as compared to the mildly affected mdx and normal mice. The results were compared to histopathological findings. MRI showed increased intermuscular fat and higher muscle T2 in the three dystrophic mouse models when compared to the wild-type mice (T2: mdx/Largemyd: 37.6±2.8 ms; mdx: 35.2±4.5 ms; Largemyd: 36.6±4.0 ms; wild-type: 29.1±1.8 ms, p<0.05), in addition to higher muscle T2 in the mdx/Largemyd mice when compared to mdx (p<0.05). The areas with increased muscle T2 in the MRI correlated spatially with the identified histopathological alterations such as necrosis, inflammation, degeneration and regeneration foci. Nevertheless, muscle T2 values were not correlated with the severity of the phenotype in the 3 dystrophic mouse strains, since the severely affected Largemyd showed similar values than both the mild mdx and worst mdx/Largemyd lineages. On the other hand, all studied mouse strains could be unambiguously identified with texture analysis, which reflected the observed differences in the distribution of signals in muscle MRI. Thus, combined T2 intensity maps and texture analysis is a powerful approach for the characterization and differentiation of dystrophic muscles with diverse genotypes and phenotypes. These new findings provide important noninvasive tools in the evaluation of the efficacy of new therapies, and most importantly, can be directly applied in human translational research.

D.P.16 Metabolic and hemodynamic alterations in the mdx skeletal muscle revisited using multi-parametric functional NMR

Abstract The mdx mouse remains a popular model of the Duchenne muscular dystrophy. While criticized for the mild impairment of muscle function and structure, the mdx remains highly relevant in terms of energy and microvascular control disturbances associated to muscle dystrophy. In this study, a non-invasive multi-parametric functional NMR approach (mpf) was used to revisit these anomalies. This NMR approach interleaved arterial spin labeled imaging and 31P spectroscopy to measure perfusion, capillary oxygenation and muscle energetics of the gastrocnemius of C57Bl/10wt and mdx mice (between 10 and 65weeks of age). Hind-limb ischemia (30min) or repeated bouts of exercises (2min) and recovery were investigated. After ischemia, phosphocreatine (PCr) depleted more in 28-week-old mdx vs wt mice (ΔPCr: 82±6% vs 62±10%). Resynthesis of PCr at reactive hyperemia appeared prolonged ( T Cr: 88.7 ±21.5s vs 53±23.3s). In parallel, vasodilatation lasted less at reactive hyperemia in mdx mice compared to wt mice. Similarly, a shorter reperfusion profile was visible in 65-week old mdx mice following exercise, confirming a default in maintaining vasodilatation in old mdx vs old wt mice but also compared to young mdx mice. Moreover capillary oxygenation at reactive hyperemia showed reduced O 2 extraction in mdx mice. In this study, the mpf-NMR protocol proved again its capacity to characterize in vivo functional anomalies, this time in dystrophic muscle. The underlying mechanisms require further investigation, with neuronal NO synthase (nNOS) deficiency most likely playing a direct role in the perfusion profile alterations. The defective nNOS might also contribute indirectly, through PFK inhibition, to the faster PCr depletion during ischemia, which could be indicative of impaired glycolysis. On the other hand, the slower PCr resynthesis and lower oxygen extraction help to appreciate and quantify the real impact in vivo of mitochondrial dysfunction in mdx.